Another weekly update by Dave Dixon of the charts on the relative rates of cases in the vaxed, unvaxed, and previously infected. Please note as usual that the source DOH data has issues that the agency won’t fix, among which is the overlap between prior infection and vaccination. But you can see that notwithstanding the emergence of Omicron, both vax and prior infection appear to limit infection risk. Dave’s notes:
1. Since 11/2/2021 MDH has published Covid reinfection data on the Minnesota Situation Update for COVID-19 web page: https://www.health.state.mn.us/diseases/coronavirus/situation.html Reinfections are defined as a person testing positive for Covid more than 90 days after a prior positive test.
“Confirmed” reinfections are defined as a positive PCR test following a positive antigen test. Probable is defined as a negative PCR test following a positive antigen test. It is implied, similar to Cases, that a PCR positive test is considered “Confirmed” and a positive antigen test is considered “Probable”. For purposes of this analysis all Confirmed and Probable reinfections will be included.
2. The weekly Covid reinfection rate displayed on the chart is calculated as the number of weekly reinfections (confirmed and probable) divided by the cumulative Total Positive Cases 90 days prior to the start of the week being considered, and the result multiplied by 100k to yield the reinfection rate per 100k per week. This is the red curve on the chart.
3. The Fully Vaccinated Age Adjusted Case Rate Per Week (green curve) and Not Fully Vaccinated Age Adjusted Case Rate Per 100k (blue curve) are taken directly without modification from the Vaccine Breakthrough Report https://www.health.state.mn.us/diseases/coronavirus/stats/vbt.html from the data file for Cases, Hospitalizations, and Deaths Over Time graphic (vbtaarates.xlsx linked on this page).
4. Data is plotted starting 5/2/2021, the earliest date we have breakthrough data. Reinfection data goes back to 6/28/2020, but the earlier data is more variable possibly due to the smaller population of previously infected people.
5. We have no knowledge of what age adjustments DOH performs in order to calculate the Age Adjusted Rates, and we have not yet submitted a formal query to MDH for this information.
6. It is likely that there are people who may have contracted a mild enough Covid infection that they did not seek medical attention or get tested, causing an undercount of the true number of breakthrough infections, non-breakthrough infections, and reinfections. We do not know how thorough and comprehensive MDH’s process is in identifying reinfections or breakthrough infections.
7. The main purpose in generating this chart is to try to compare the relative infection rates for previously infected people to vaccinated people, and then to unvaccinated people. We can see that throughout the Summer of 2021 the three rates were all low, during the time period in Minnesota when Covid cases were very low in general. However, unvaccinated infection rates were always the highest of the three, and for several months the reinfection rate was higher than the vaccinated breakthrough rate. For the week starting 12/05/2021, the most recent date where we can be considered to have complete breakthrough case data, the reinfection rate was 86.6 per 100k, the vaccinated infection rate was 257.1 per 100k, and the unvaccinated infection rate was 927.3 per 100k. From these rates we might say that the vaccinated are 3.0 times more likely to test positive than someone who previously recovered, and the unvaccinated are 3.6 times more likely to be infected than the vaccinated.